Equipment and methods for aligning and splicing silica based optical fibers have been developed and improved for many years. It is well known that core/cladding eccentricity, cleave angle, curl, fiber-end contamination and mode field diameter (MFD) mismatch are the main reasons of loss appearing in fusion splices of optical fibers.
Generally, an exact knowledge of the mode field diameter (MFD) of an optical fiber permits determination of various propagation characteristics of light waves guided by an optical fiber, e.g. the coupling efficiency between a light source and an optical fiber, splice losses and chromatic dispersion, etc. The knowledge of the MFD of an optical fiber is usually obtained by the so-called transmitted near field (TNF) method. In this method, light is injected at one end of the fiber and either a magnified image of the other end of the fiber is scanned by a movable detector, or the fiber-end is moved while the detector is stationary. The major disadvantage of the TNF method is that the method is destructive since the fiber has to be cut in order to access the end surface where the MFD is measured. Therefore, the method may not be applicable to handle MFD measurements in dynamic processes, e.g. monitoring variation of MFD during fusion splicing. Furthermore, expensive instrument and highly qualified operators are needed to perform TNF measurements, which are not always available, especially not in the field.
The MFD can generally be considered a function of the diameter of the fiber core, among a lot of other parameters, as appears from e.g. Swedish patents 502374 and 502290. As disclosed in these patents, an image of a heated fiber is captured and analyzed for finding a value of the fiber core diameter. A similar method is disclosed in published Japanese patent application 2000 275469.